If the man says to his wife over the phone, “You are divorced” or “I divorce you,” then it counts as one divorce (talaaq), just as if he said that in her presence, or he said it in her absence without using the phone.
Similarly, if he writes a message on the mobile phone addressed to his wife in which he says that he has divorced her, or tells someone else that he has divorced his wife, and he intended to divorce her, then that also counts as a divorce.
For more information please see the answer to question no. 72291.
And Allah knows best.